Welcome to PhysLink.com - Your physics and astronomy online portal. Stay a while! Check out our extensive library of educational and reference materials. Also, check out our fun section!
Since magnetic forces can do no work, what force IS doing the work when a bar magnet causes a paper clip to jump off a table and stick to the magnet?
Asked by: Steven Leduc
The original assumption that a magnetic field can do no work is incorrect. A magnetic field has an energy density that is equal to the magnetic induction (B) squared divided by twice the permeability (mu sub zero). If you were to sum (integrate) this energy of the magnet over all of its field before it picked up the paper clip and compared it to the same sum after you picked up the paper clip, you would discover that there was a loss of field energy. The paper clip has in effect 'shorted out some lines of magnetic flux'.
How much energy was lost? If you took hold of the paper clip and pulled it out to such a distance that the magnetic pull was insignificant, the work you did in this process would exactly equal the amount of energy lost when the clip was on the face of the magnet. When you picked up the clip with the magnet the clip was accelerated toward the magnet acquiring kinetic energy. This kinetic energy will equal, ignoring air drag, the loss of magnetic energy in the field. This kinetic energy will be dissipated in the form of heat on impact of the clip with the magnet.
For further understanding of the energy in a magnetic field, you may want to study magnetic fields in solenoids. See the Reference below.
Physics, Volume 2 by Halliday and Resnick
Answered by: Robert Gardner, M.S., Retired Physicist
Fireflies In My Room
Today's Price: $19.00
Light up your room with magical fireflies with this great set! Buy it now! Deal ends at midnight!
Here are our physics & astronomy bestsellers:
Solar Science Kit
Mini Plasma Ball
Deluxe 3D Mirascope
Deluxe Water Rocket Set
Revolving Multi-Color Fiberoptic Light
Magnetic Levitator - Classic
Ultimate Chemistry Set CHEM C3000 v2.0
Worlds Strongest Magnets
Solar System Planetarium 4M Kit
Blue Fiber Optic Light